Buick is doubling the number of cars it offers with a hybrid powertrain with the introduction of the 2012 Buick Regal eAssist hybrid. The more efficient Regal is expected to be rated 26/37 mpg city/highway pending EPA certification. The eAssist Regal is debuting at the 2011 Chicago Auto Show.For those of you keeping track, that's one mile per gallon more in the city for the Regal eAssist hybrid than Buick predicts the LaCrosse eAssist hybrid will achieve in the EPA's hands. Both cars feature a 115V lithium-ion battery and a 15-kilowatt motor/generator. The engine at the heart of the cars is a 2.4-liter four-cylinder engine that produces 182 horsepower on its own, and is mated to a six-speed automatic transmission.The same 17-inch lightweight alloy wheels (and low rolling resistance tires) are used on both Buicks. Thanks to the Regal's smaller size, however, the wheels don't appear as undersized as they do on the LaCrosse. If the driver is in an especially green mood, the car can be put into a fuel economy optimizing mode with the AC unit that allows for more frequent and longer engine stop periods.Regenerative braking is, of course, part of the package. Unfortunately, so is reduced cargo capacity. Trunk space on the hybrid Regal is 11.1 cubic-feet (down from just about 14 cubic-feet on the regular Regal), still a smidge bigger than the LaCrosse eAssist's 10.9 cubic feet.The Regal with the eAssist technology goes on sale this fall, but it's unclear where Buick will price it, given the LaCrosse is expected to start around $30,000. We'll have more details soon, live from the Chicago show floor.